power help with TK 740

[tunnelmot]

Hi all,
Digging through my attic, and found my old Kenwood TK 740-( LTR/conv. uhf). I am taking an interest again in UHF/LTR stuff. And it would be a nice treat to get the screen glowing again and start playing with this radio once more. Kenwoods were once a major part of my "arsenal", then put up for many years.

My problem- Stupid mistake a long time ago, power was applied in reverse, w/o proper fusing. Won't power up, when power is applied, it shuts down power power supply/blows fuse.

My question-Can it be resurected?

? 1- Can anybody send me a schematic?
? 2- Has anybody dealt with this SAME problem and tell me what component(s) to replace. (I have done pcb repairs before/decent solderer). If I have solid info/pictures, I can replace a diode/ resistor/ etc.
? 3-Should I look for another TK family mobile, can the board be replaced? If so, what components?
? 4- Is this a lost cause, should I just buy another one? Does the reverse polarity issue also blow chips, eeproms and such?

Would like to get this thing going again, but no big deal, just seeing if I can repair it to bring it back to life.

Searched around on the net a little, and have not found any solid info yet.

RICH

 

[rescuecomm]

Some radios have a large diode on the pc board where the power leads come in from the case. If you shorted this with excess current, it could be the culprit. If that is not the problem, the radio is probably beyond economical repair. Search online stores and hamfests for another?

Bob

 

[Thayne]

Rescuecomm is right, the diode probably shorted with wrong polarity. you can test this by cutting or unsoldering one end and see if it powers up; but you should get it replaced with the proper diode or next time it will toast more expensive things or blow the trace right off the board. Be sure it is fused no more than 10 amps too.

 

[tunnelmot]

Actually a TK 840

I get my "TK"s mixed up. It was actually a TK 840.

I will try the diode trick and see what happens. I still have not found the schematics, and as a last resort, I will just search the 'bay for another one. I am hoping to get it fired up again now that I have the software. We'll see how it goes.

It was a great radio, but don't want to break the bank getting back into LTR if I can repair over a few evenings.

Thanks, we'll give it a shot,
Rich

 

[tunnelmot]

Worked! Thanks!

Thanks guys,
Clipped the diode, applied 3A fused power, and IT LIT RIGHT UP! Even still has my old LTR programming in it!

Next time I head to Fry's or Rat Shack, I'll pick up some diodes and commence the hot-tipped surgury.

Any ideas on what would be a safe value?/rating for the diode. Still no luck on schematics, and saw no obvious id's (like bands on resistors).

Either way, thanks guys. Digging out that old radio and not being able to get it working was nearly keeping me up at night.

Rich

 

[Kcomm]

The reverse current diode in a tk-980 is a dsm3ma1. Cant locate an 840 book, but this'll work.